The treatment of metal surfaces for the purposes of cleaning, and so as to allow for the subsequent treatment and coating, preferably by vaporization of the thus cleaned and treated surfaces, have long been known and various mechanical and/or chemical means have been proposed to carry out this cleaning.
It has been found in many cases, and in many applications, that such mechanical and/or chemical cleaning processes are either ineffective in achieving the required degree of cleaning, or involve the use of expensive and complicated equipment and may, in fact, give rise to damage to the surfaces being treated.
It has been proposed to clean the surface of a metallic object by subjecting it to a vacuum arc discharge wherein the metallic object constitutes an effective cathode. Such proposals have, for example, been made in U.S. Pat. No. 4,534,921, British Patent Specification No. 2086788 and in a paper by V. E. Bulat and M. Kh. Esterlis--"Removing Scale, Oxide Films and Contaminants from Metal Components by Vacuum Electric Discharge", Fizika i Khimiya Obrabotki Materialov, Vol. 21, No. 3, 1987, pp 49-53.
In this paper, it was explained that such a vacuum arc discharge takes place between the anode and discrete locations on the cathode known as cathode spots, and that these spots move at random on the surface of the cathode. The phenomenon of the production of cathode spots in vacuum arc discharges in general has been described in detail in "Vacuum Arcs: Theory and Application, J. M. Lafferty, Editor; Wiley 1980".
The nature and characteristics of the cathode spots in vacuum arc discharges have been described in some considerable detail in "The treatment of metal objects by electric erosion" by I. Nikoshevitz, Nauka Technico, Minsk 1988. In this book it is suggested that the cathode spots can be considered as falling into two main categories, namely:
(a) spots having a mean diameter of more than 1 mm and which travel at a mean speed which is less than 100 cm per second--such spots being hereinafter referred to as "large, slow-moving (LS)" cathode spots, and PA1 (b) spots having a diameter of less than 1 mm and travelling at a speed greater than 100 cm per second--such spots being hereinafter referred to as "small, fast-moving (SF)" cathode spots.
These characteristics of size and movement of the cathode spots has also been investigated inter alia by the present inventor, and this in the context of the voltage current characteristics of the vacuum arc discharge. Thus, it has been shown that the voltage current characteristic can consist of successive rising and descending portions, i.e. portions having positive and negative gradients. The inventor has shown that in the initial stages of the arc discharge, and with a small anode-cathode spacing, the arc extends from a single or a very limited number of cathode spots to a very limited area of the anode. Increasing the arc current is accompanied by an increase in the arc resistance and, as a result, it is accompanied by an increase in the arc voltage. In consequence, in this initial stage of the arc discharge the arc voltage current characteristic has a positive gradient.
When, however, as a consequence of further increase of arc current, the number of cathode spots increases and, as previously indicated, they move randomly around the entire cathode surface, the volume of the arc increases substantially and, as a consequence, the vapor density in the arc decreases significantly and with it the arc resistance. As a consequence, the arc voltage current characteristic passes into a negative gradient portion.
If now, however, the vapor density in the arc rises beyond a specific critical value, for example by a continued increase in the arc current and consequent increase vaporization of the cathode surface, it has been shown that the arc voltage current characteristic passes once more into the positive gradient region.
It has furthermore been shown, inter alia by the present inventor, that passage of the voltage current characteristic from the negative gradient portion to the positive gradient portion can be achieved by ensuring that the volume of the arc discharge, or its cross-sectional area, does not exceed a certain critical value seeing that this carries with it the consequence that the vapor density in the restricted arc volume is relatively high. In other words, this transition from the negative to the positive gradient for the voltage current characteristic is achieved with increasing arc current whilst maintaining the arc cross-section substantially constant or being reduced.
Now in view of the fact that the action of the arc on the surface to be treated takes place invariably in the region of the cathode spot, the fact that the cathode spots move randomly over the surface to be treated must give rise to a non-uniform treatment of the surface.
An attempt to overcome these problems arising out of the random movement of the cathode spots on the cathode surface is described in European Patent Application No. 0 468 110 A1, wherein there are disclosed means for directing the movement of the cathode spots on the cathode surface in a desired direction. It is, however, repeatedly stressed in the European patent application that the arc which is generated has a voltage current characteristic having a negative gradient and it is furthermore clear that these negative gradients arise in view of the fact that the mode of applying the arc voltage between the cathode and anode is such that the arc occupies a maximum volume with a correspondingly maximum cross-sectional diameter.
It has now been discovered that treating a cathode surface with an arc discharge having such negative gradient voltage current characteristics carries with it certain distinct disadvantages, among which are the facts that the treatment is time-consuming and uneconomical.
The present invention, on the other hand, is based on the surprising discovery that when the surface to be treated is subjected to a vacuum arc discharge having a positive voltage current gradient and with an arc current sufficiently high (greater than 50 amperes), an increased efficiency of surface treatment of the cathode surface can be achieved.